text:
What is the general mathematical relationship between increasing an airplane's weight and the needed thrust to fly?
I understand that increasing an aircraft's weight without changing any other properties means that its takeoff, stall and cruising speeds all increase since you need to generate additional lift.  So of course that means a plane needs to generate more thrust to achieve these higher speeds.  But I'm trying to figure out what exactly is the mathematical relationship?  If I make the plane 20% heavier will I typically need about 20% more thrust?

I will only answer part of your question, that may be enough to help form the rest of your intuition.  In the left formula, lift increases with the square of the speed, all other factors equal.  But put the other way that means speed increases with the square root of lift required.  So a close-enough approximation for speeds would be the speed increase with the square root of weight.  If you increase the weight of the airplane by 120%, your stall speed increases by 109.5%.  This doesn’t account for the increase in drag at the higher speed, so thrust would also be a non linear relationship.

@Max Drag (and therefore thrust required) goes up with the square of speed, so does that cancel out your square root?

@StephenS It's not quite that simple.  (a) I may be at a slightly different angle of attack and C(L) and C(D) are specific to an AOA.  (b) I think it is "intuitively correct" to think of D = T.  But 99.9% of the time I will be flying with some positive angle of attack and thus my thrust vector and drag vector are not colinear.  In fact in a climb, my propellor is contributing a meaningful vertical component to counteract gravity.  At any positive angle of attack I am generating slightly more thrust than what is contributing to horizontal motion.

While the takeoff and stall speeds increase, the cruising speed doesn't need to change. Assuming that the cruising speed is kept constant, the thrust increase can be derived as follows:
At cruise, the aerodynamic lift $L$ (which depends on the air density $\rho$, the cruise speed $V_C$, the projected wing surface area $S$ and the lift coefficient $C_L$) is equal to the weight $W$ (which depends on the mass $m$ and the gravitational acceleration $g$).
\begin{equation}
    L= \frac{\rho}{2}\cdot V_C^2 \cdot S \cdot C_L = m \cdot g = W
\end{equation}
At cruise, we can also assume that the thrust $T$ is equal to the drag $D$ ($C_{D0}$ and k are two constants).
\begin{equation}
D = (C_{D0} + k \cdot C_L^2) \cdot \frac{\rho}{2}\cdot V_C^2 \cdot S = T
\end{equation}
Joining both equations to get rid of $C_L$, we get the required thrust to fly with a certain mass:
\begin{equation}
T = \left(C_{D0} + k \cdot \left( \frac{m \cdot g}{\frac{\rho}{2}\cdot V_C^2 \cdot S} \right)^2 \right) \cdot \frac{\rho}{2}\cdot V_C^2 \cdot S
\end{equation}
Calculating the derivative results in the answer to your question:
\begin{equation}
\frac{\mathrm{d}T}{\mathrm{d}m} = \frac{4 \cdot k \cdot g^2}{\rho \cdot V_C^2 \cdot S} \cdot m
\end{equation}
The right side of the equation depends on the mass $m$, which means that the relationship between the mass increase and the thrust increase required to maintain the same cruising speed is not constant.
Instead of increasing the thrust, you could fly with the same thrust and a lower cruising speed. Another option would be to fly at the best endurance speed. In all cases, you can derive the desired relationship similarly.

Nice derivation! Yet another option is to fly at a different altitude as weight changes, which is what airliners typically do. This would result in changes to $\rho$ in the equation.

This is showing thrust required scaling with m^2, but other answers are suggesting that typically it is about linear.  Are you maybe holding something constant that isn't required, like angle of attack?  Or is it purely down to holding ρ constant, flying in thicker air when heavier to get more lift from the same wing area and (true?) air speed?

@PeterCordes It depends on the constraints. If you assume $C_D/C_L$ to be constant, it is indeed linear. For this to be true, you would need to cruise at a different speed and angle of attack after increasing the mass. As I wrote at the end, we can make many assumptions, and I've only derived it for the case where the cruising speed is kept constant.

I see.  So same speed in same air density, requiring a higher angle of attack and thus much more drag, scaling with $C_L^2$; that's where the quadratic scaling comes in.  Interesting that it's so much costly to get more lift at the same speed.

Interesting.  I guess I should have asked my question with more specifics.  The only thing I was clear wouldn't change is the other properties of the plane.  In my head I was thinking about a change in the minimum thrust needed to cruise.

@J-bob In that case it is linear. You can fly with minimum thrust at $C_L=\sqrt{C_{D0}/k}$, which means that $C_D/C_L = cte.$.

For the majority of planes and flights in practice, it's untrue that cruise speed increase with weight. Heavier flights would fly a higher AoA instead, maintaining the same cruise speed.
Every plane has a (mostly) constant lift/drag ratio (approximately the same as glide ratio) at cruise speed and altitude. In a straight and level flight where lift=weight and drag=thrust, the statement that "20% heavier flight needs 20% more thrust" isn't far from the fact at all.

This is a very interesting question!  And the simple answer is...
$$T = W \frac{C_D}{C_L}$$
where $T$ is thrust in lbs, $W$ is weight in lbs, and $C_D$, $C_L$, are coefficient of drag and coefficient of lift, respectively.  Of course, this simply explains that the thrust is in direct relation to weight.  We are, of course, discussing conditions for an aircraft in flight. So if the weight is increased by 20 percent, the thrust must also increase accordingly by 20 percent.
Keep in mind that the coefficient of lift and coefficient of drag are affected by atmospheric density.  My supposition is that you have a handle on those already. And the conversions to metric can be done anytime.
But, you say, maybe you would like to know thrust horse power...  also given here in English units.
$$T_{HP} = \frac{T\cdot\ V}{550}  =  \frac{W V}{550}\frac{C_D}{C_L}$$
where $V$ is velocity in feet per second. Now we can see, for instance, that increasing the weight and/or the velocity will increase the thrust horsepower required to maintain flight.
Just some brief insights on this question, a very good question, indeed.


text:
Where does an FMS wind information comes from to calculate V approach?
To calculate the V approach the FMS is using a wind either taken by the ADIRS or inserted on APPR PERF page. At first I will say that it takes the wind inserted by the crew, but I am not completed sure about this assumption since the MINI FUNCTION on A320 will need as well wind info taken by ADIRS to be able to cope with sudden loss of headwind / increase tailwind.

It uses the wind entered in the PERF APPR page to correct the computed VAPP:

Takeoff, performance and go around speeds
[...]
The FMGC uses the performance model and either the predicted landing weight or the
current gross weight at transition to the approach phase to compute approach speeds
(VLS, VAPP, F, S, Green Dot). On the PERF APPR page, the selected LDG CONF determines
the applicable VLS and VAPP, the latter being updated by the WIND correction that the pilot
enters on the same page.

(A320 FCOM - Autoflight - Flight Management - Performance, emphasis mine)

I'm a bit disappointed. I imagined the aircraft was able to evaluate wind component based on difference between trajectory as defined by inertial sensors and trajectory as expected by air mass sensors (static and dynamic pressure, AoA, side slip sensor,...)

@ManuH The aircraft is capable of doing exactly that and it will display this wind component on the ND (Navigation Display). You don't want to use this current and local wind for VAPP though, because this should be based on the wind on the ground at the airport, which you can only know in advance from the ATIS. If you slow down to VAPP at 1000ft you still have very different wind as measured by the aircraft compared to ground level.

The approach speed that is shown on the MCDU performance page is based on the pilot entered wind or in Airbus terms, tower head wind component. The headwind component is the one which affects the aircraft energy on the approach.
The approach speed also known as Vapp is calculated as follows:

Vls (lowest selectable speed or Vref in landing config) + 5 knots (for autothrust) + 1/3 headwind component (limited to a maximum of 15 knots).

The ground speed mini (GS mini) function uses this calculated Vapp as a minimum approach speed to vary the approach speed with changes in headwind component. And yes, you are absolutely correct. The live wind direction and speed data is fed to the FMGC by the ADIRS. From this data, instantaneous headwind is computed by the computers and the approach speed on the speed tape is varied. The autothrust then adds or reduce thrust to keep the target speed. The GS mini uses the aircraft inertia to vary the approach speed. For example, if there is an increase in headwind, because of inertia, the aircraft tries to increase its indicated air speed to prevent a reduction in ground speed. This change in speed then becomes the new Vapp and thrust is added to maintain the Vapp. The idea behind the GS mini is to conserve the aircraft energy. If there is a headwind and if it suddenly dies out, the speed reduces and a thrust increase is required to keep the speed from going close to the stall speed of the aircraft (Vref is just 1.23 x stall speed in landing config). When Vapp is increased with an increased headwind component, even when the wind speed goes down, the aircraft is maintained at a high energy state. The target speed then is changed to a lower value and thrust is reduced to match the new wind conditions. It is a very interesting sight to see the aircraft Vapp target moving on the speed scale of the PFD on the approach. This is very noticeable in windy approaches.

In a conventional aircraft, when we fly gusty approaches, we add about 10 knots to our Vref, in order to compensate for any reductions in wind speed. The GS mini is a more accurate version of this as it varies approach speed based on the actual prevailing winds. For GS mini to be active, the pilots must use managed approach speeds and autothrust must be engaged.

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text:
How to sign hours on Logbook if doing aerial work in right seat on single pilot aeroplane?
Recently I got an offer to work during the incoming summer as survey pilot, but since I do not have the required hours for insurance I can not be the PIC on left seat. So they proposed to be on the right seat until getting enough experience and then switch on. Can I sign those hours on the right if the aircraft is certify for single pilot operation? How?

This is a legal question. You cannot expect reliable answers for that here, only personal opinions.

@TimothyTruckle questions about regulations are on topic here, such as What are the requirements for logging SIC time? (FAA)

Also, I noticed you tagged "EASA regulations" so I assume that you don't care what the FAA has to say about the matter.  Is that correct?

The owner can name you as a pilot if he really wants you.  Not sure if that works in Europe though.

Well, I am going to work in Europe under EASA rules, for that reasons I tagged EASA regulations. But if you got some info for FAA as well could be interesting to share some light.

Are you asking if a) you can fly from the right seat with another pilot in the left seat, or b) fly solo from the right seat?

What makes you think which seat you're in changes whether you can be PIC or not?

@Pondlife a) fly from the right seat with another pilot in the left and log PICus.

I am not sure about EASA regulations but in the US you can log PIC time whenever you are the sole manipulator of the controls and properly rated to fly the aircraft.  In the US there is a distinction between the legal PIC (person legally responsible for the flight) and PIC time which is the person actually flying the airplane.  Since the plane is certified for only single pilot operations you can only log PIC time not SIC time and only when you are the sole manipulator of the controls.  The FAA regulations for logging time can be found in FAR 61.51.

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text:
Is there an FMC not connected to the AP/FD?
Is it possible to have an FMC to compute the performance and navigation without having it linked to an autopilot or flight director?
Does/did such setup exist?

Yes.  For may overwater operations a third FMS/FMC is required as a backup to the first two.  That one is typically not connected directly to the AP/FD but a simple switch will allow it to replace the failed system.  On that FMS, you can do anything you want including performance calculations.

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text:
Does EASA approve the use of localizer back course?
Does EASA approve the use of localizer back course? Are there some countries in Europe that allow it?

This would likely be only seen on specific instrument approaches/ILS setups, not as a blanket approval.  Is there a specific airport you're curious about?

Some countries do not allow the use of the ILS back course anywhere on their territory (e.g. UK) and I heard that EASA in general do not approved this use of the ILS so I was wondering if some countries do not follow the EASA rules on this on general perspective.

Take a look at this forum discussion

Depends what you mean. If you mean "are they approved at EASA airports?" I can't help except to say that I have never seen a plate for one, flying into most EASA countries into a wide variety of airports.
If you mean "are EASA aircraft and pilots allowed to fly them in countries that do allow them?", I can tell you that in the Radio Navigation syllabus for the EASA ATPL exams there is a learning objective that the student must know what to set on an OBS or HSI to fly a back-beam localiser approach (if you want to check it's LO 062.02.05.02.08 in the new syllabus, 062.02.05.02.13 in the old one which is just about to go out but more widely available at the moment). This implies that flying one in an EASA-registered aircraft with an EASA licence and IR is not prohibited.

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text:
Where can I find the specific power output of a specific NDB?
As a rule of thumb I can get an approximate range of an NDB with the formulae:
Over sea: range=3SQRT Power
Over land: range=2SQRT Power
To use this formula I need to find out what Power has the NDB that I need to use; Where should I look to find out?

For what it's worth, you have a first indication of the power by looking at the NDB class (e.g. MH is less than 50 W and usable within 25 NM at all altitudes). Note that the power which must be taken into account is the effective isotropic radiated power (EIRP), which depends on the antenna gain.

@mins  Well it's a start. But I was wandering if there is a list of NDB specifications somewhere on AIP or elsewhere open for the pubblic to use.

I'm not even sure if the US has more than 10 NDB's left, and I would bet some of those are long dead. I don't remember the last modern aircraft I've seen equipped with an ADF. Then again, there's plenty of AM radio stations in the US and many of them are operating in the 50 kW power range. I remember getting in trouble as a student pilot on my long XC because of weather, and using the NDB in Puerto Rico to make sure I was still on course, 'cause I couldn't see the shoreline and I was worried. Very worried. LOL!


text:
How airplanes of new generation communicate with ATC over remote areas? (North Atlantic Region)
How new airplanes get in contact with ATC while flying in remote areas? Do they use VHF datalink (VDL) ?

They use HF radio.

Related


text:
Is there any airport planning to install ILS CAT IIIc?
I know that right now no airport has an ILS CAT IIIc, but is there any airport in the world planning to have one in a near future?
Otherwise why bother to have rules and specifications for something not even planned to be in future?

The technology required for implementing CAT IIIc approaches does not yet exist. Specifically, taxing in 0 visibility is not possible.

Related: Are zero visibility landings practical and desirable for commercial aviation?

@J.Hougaard considering the fact that we currently have self-driving cars that have to navigate streets full of other, barely controlled moving things, applying that same technology to aircraft, which only move on the ground in very controlled situations, shouldn't be too big a stretch. I would say it is possible, it just hasn't been done yet.

@FreeMan We're saying the exact same thing, just using different words.

Sounds like we have the required technology (scientific knowledge and engineering experience) for it to be possible (to implement after further R&D), but not the required technology (actual software, hardware and procedures) for it to be possible (to implement without further R&D).


text:
If the static port get blocked, and no alternate static air source is fitted, will the transponder keep provide mode C?
If the static port get blocked for any reasons and the aircraft is not fitted with an alternate static source while in Mode C with SSR, will the ATC still get the height information from my transponder?

how does it know that the port is blocked? So I'd say: yes....

I remember something about transponder with mode C which is mechanically actuated by the altimeter's aneroid capsule so I guess it will give erroneous information. But I will wait an explanation from someone who knows more about this. That's all I've got.

Related: https://en.wikipedia.org/wiki/Aeroper%C3%BA_Flight_603

It will stop reporting correctly as it is connected to the same static air line as the altimeter

The transponder depends on an external altitude encoder to provide the digitized pressure altitude that the transponder needs for its Mode C replies. The encoder is connected the same static air line as the altimeter and is wired electrically to the transponder

However the transponder may not know the port is blocked and will still report the erroneous information it is receiving. If the port is blocked it’s pertinent to shut off altitude reporting manually.

yes if you see the malfunction then shut it off. however in most cases ATC will ask for your altitude when the mode-c altitude does not match your assigned altitude. but you will also have trouble with the main altimeter and airspeed indicator. It may be most noticed with the behavior of the vertical speed indicator. Also most GPSS units have an option to show geometric altitude, which will show gross error in the altimeter, but not reliable for small errors as it is not compensated for pressure variations.


text:
How to select a SID during the planning stage?
During the planning stage of an IFR flight how to choose the correct SID for your route in an airport with multiple runways and multiple SIDs? and how to predict the runway you will be going to use?

"File what you want, fly what you get."
There's no way to know for sure what clearance ATC will give you until you actually call them. If you're departing from an airport that you know well then you may be able to make an educated guess but that won't help if the winds change, an incident closes a runway, or any number of other things happen to invalidate your assumptions.
For planning purposes, just use the SID that best fits your needs, based on all the information you have available: direction of departure, aircraft type, weather, local procedures etc. But ultimately, you'll have to fly what you get (or what you can negotiate with ATC).

Picking one which gets the departure fix that you would like is about the best you can do...

Which runway do you plan if the airport has many of them and you are not familiar with it?

@AndreaGhilardi, usually the SID chart has a route from each runway to a common waypoint. So you chose your preferred departure waypoint and leave it up to the tower to tell you which runway is active for departures.

The runway(s) in use will depend primarily on the winds at the time, though there are other factors such as maximum arrival/departure rate that may take precedence, especially if the winds aren't very strong. It's disruptive to traffic flow to change the active runway, so when the winds change, they will often wait until there's a natural lull in the traffic or a pilot rejects the assigned runway(s) because the crosswind or tailwind component exceeds their limitations (which varies by aircraft and pilot skill).
You can also request to use a specific runway. For instance, pilots may accept a crosswind or even a tailwind to get a runway with the shortest taxi time to/from parking, or allows a straight in arrival or straight-out departure, or is longer than the one aligned with the winds to provide more safety margin, etc.  Whether ATC grants such a request will depend primarily on the manageability of potential conflicts with everyone else following the normal plan, which mostly boils down to how much traffic they're juggling at the moment.
Regarding the SID, usually there's either one generic SID for all departures, or there's a set of SIDs (or one set using VORs and another using RNAV) divided up by direction so only one of them would makes sense for your flight.

That can be selected based on whether the SID is intended for use by your type of aircraft, as well as your aircraft’s performance capabilities and onboard avionics (eg an RNAV SID is useless unless your plane has RNAV equipment aboard or can’t meet the minimum required climb rates, etc.).  As stated above, depending on your aircraft type and airport, ATC may or may not clear you to fly that SID.  EFB apps like ForeFlight do have the capability to show previously cleared routes so you can get a sense of how ATC likes to handle airplanes for those routes.  Also be aware of IFR Preferred Routes or TEC routes on the east and west coasts which will be often used by ATC and will influence what DPs you choose.


text:
How do aerodynamic loads explain the Spaceship2 accident?
From the NTSB's preliminary report, we read in the "Findings" section:

Although the copilot made the required 0.8 Mach callout at the correct point in  the flight, he incorrectly unlocked the feather
  immediately afterward instead of waiting until SpaceShipTwo reached
  the required speed of 1.4 Mach.
The unlocking of the feather during the transonic region resulted in uncommanded feather operation because the external aerodynamic
  loads on the feather flap  assembly were greater than the capability
  of the feather actuators to hold the assembly in the unfeathered
  position with the locks disengaged.

Taking into consideration that, as reported elsewhere:

The vehicle's two tail wings are supposed to move when the spacecraft
  hits a certain speed, a maneuver known as "feathering." This
  repositioning helps to increase drag in order to slow the vehicle down
  for reentry.

From the article above, let's also take note of the visual depiction of such configuration change:

How is possible that this configuration change, meant to increase drag, needs a "preventing" (for lack of a better word) mechanism?
Since the repositioning helps to increase drag, how do the aerodynamic loads make the repositioning occour?
Wouldn't the (additional) drag on the tail section make it go back down again?

Not a full answer as I don't have any technical info: my assumption is that the tail was either generating lift or vibrating in part of the transonic region, or that normal use of the control surfaces in pitch would have this effect. (I'm assuming the elevator surfaces are on the boom section though.)  Also the report mentions inertial loads, which might imply vibration?

Watching the video, it seems to me like it's more that the fuselage pitched up, the tail wings stay mostly level.

Good find by @2012rcampion - the boom section remains approximately level and the body starts to tilt up, quite slowly (at first). So lift from the boom's surfaces seems unlikely. I think they would be starting to pitch up at approximately this time in the flight profile, by the way. I wonder if they would expect to be established in the climb, and at lower air density, at the preferred unlocking point.

How do aerodynamic loads explain the Spaceship2 accident?

Aerodynamic load overpowered the actuators holding it in place, forcing it to open and causing SpaceShip2 to break up in flight. Although the rudders were unlocked early when SpaceShip2 was going at Mach 1.0, this action alone should not have been enough to pivot the tails upright, because neither pilot took the further step of turning the feather handle to actually move them. However, the tails rotated upward anyway, and the increase in drag at this point in the flight caused it to crash.

How is possible that this configuration change, meant to increase drag, needs a "preventing" (for lack of a better word) mechanism?

The feather is ultimately there for re-entry. It acts as a hinge and can go up and down. This system increases atmospheric drag on the spaceship so that it naturally positions itself during re entry. To make sure the tail doesn’t move forward early, SpaceShip2 has a structural lock that holds feather in place. Around Mach 1.4, the lock is moved so that the feather is now mechanically free. It won’t move though because at Mach 1.4, aerodynamic forces keep it nailed back. In addition to the lock, the feather itself has a big actuator that drives it up and down. So just because it’s unlocked doesn’t mean it’s just flopping in the wind and it can go where it wants. In the (initial) boost phase, the aerodynamic forces can overcome that, which is why it is  locked .

How do the aerodynamic loads make the drag naturally increase?

Aerodynamic forces are generated by the difference in velocity between the spacecraft and the air. There must be motion between the spacecraft and the air. If there is no relative motion, there is no lift and no drag. In order to maintain a constant airspeed, thrust and drag must remain equal, just as lift and weight must be equal to maintain a constant vertical speed. If in level flight, the engine power is reduced, the thrust is lessened, and the aircraft slows down. As long as the thrust is less than the drag, the aircraft continues to decelerate until its airspeed is insufficient to support it in the air.
Calculations can be found here
Aerodynamic forces are used differently on a rocket than on an airplane. On an airplane, lift is used to overcome the weight of the aircraft, but on a rocket, thrust is used in opposition to weight. Because the center of pressure is not normally located at the center of gravity of the rocket, aerodynamic forces can cause the rocket to rotate in flight. The lift of a rocket is a side force used to stabilize and control the direction of flight. While most aircraft have a high lift to drag ratio, the drag of a rocket is usually much greater than the lift.

Wouldn't the drag on the tail section make it go down again?

There are two conventional ways to transition from space back to Earth: One is to come in as a winged airplane, like NASA’s space shuttle. In this way the spacecraft can land where it wants, but hitting the perfect angle of attack is tricky. The second method is as a capsule, which is essentially dummy-proof, but has no control over where it lands and subjects its occupants to high G-forces. SpaceShip2 is a bit of both. Its tail folds up and down, changing the vehicle’s shape from something that falls like a capsule to one that flies like an airplane once it’s back in the atmosphere.
Each of the two tails acts as a rudder, and has a small, horizontal flap at the back, extending to the outside. When the plane is descending, both tails can pivot upright, together, from 0 to 90 degrees, so that they stand “vertically” behind the plane. In this configuration, the tails and flaps create drag. When pointed upward, the tails are at right angles to the direction of airflow, creating a huge amount of drag on the vehicle, which slows it down without overheating the spacecraft. This method works because SpaceShip2 is not coming back from orbit.If put in feathered configuration during powered flight, the drag vector and thrust vectors are horribly misaligned, resulting in extreme pitch rates and, since the aircraft wasn't built to withstand those stresses, it would break apart

However, the tails rotated upward anyway, that's my question: how was that possible?
It won’t move though because at Mach 1.4, aerodynamic forces keep it nailed back. which ones? the same that made it move up at 0.8?
I think I have to rephrase the third question.
I do not feel you have answered the fourth.

@ Federico: After the spaceship reaches apogee the vehicle's twin tails rotate upward, stabilizing and slowing SpaceShip2 as it reenters the atmosphere. It's not feather deployment at high speed that is the problem, it's deployment during powered flight. In order to fly during powered flight, the thrust vector must be aligned with the center of mass and center of pressure (drag vector). Otherwise, the vehicle will tumble due to offset thrust.

I'm sorry, but I do not see how your comment addresses any of the points I raised. Adds an interesting factoid, but does not answer my doubts.

Your original question was "How do aerodynamic loads explain the SpaceShip2 accident?" I answered it in the opening para itself. In your question you need to segregate the cause of the crash from the mechanism involved in operating the spacecraft. Based on the NTSB report, so far, there is no specific clue that a malfunction occurred. The crash has been attributed to pilot error.

that's the title, the question is in the body of the post.


text:
How to calculate the best glide speed if there is none on the POH?
I am studing the POH of Seneca II and could not find the Best Glide Speed. Where can I get this information from? I thought to multiply Vs clean for 1.4 to get an approximate result but I am not sure if this is the correct way to do so.

According to the FAA, the short answer to your question is approximately "halfway between Vx and Vy."
While there is a lot of physics and calculus that can give a precise answer to this question, there is another answer that uses "pilot math". The FAA has a nice publication titled Best Glide Speed and Distance, which explains that there are two different types of "best" glide speeds.

The glide speed that gives you maximum distance covered for a given altitude (Best Glide Distance); and

The glide speed that gives you the maximum time in the air (Best Glide Time).

Per the FAA document, the glide speed that gives you the most distance covered is approximately halfway between Vx and Vy. For example, on a C172 with Vx at 53 and Vy at 73 Max Glide would be about 65. The speed provided in POHs is usually calculated at Max Gross Weight, so your actual Best Glide Distance speed will be a little lower if your weight is less than max gross.
Alternatively, if your goal is to stay in the air as long as possible, then you'll want a speed that minimizes the descent rate. This is typically slower than Best Glide Distance speed, and can be easily determined by pitching for an airspeed that gives you the lowest vertical descent speed on the VSI. There are times when you don't care how far you glide, and instead want to maximize the time in the air in order to troubleshoot. For example, if you are over an area with no suitable landing location (like over the ocean), you'll want to maximize your time to try to restart your engine. Or if you are over your landing location, but want extra time to finish your emergency checklists.

Gliding clean, with the engine at idle, you may find the IAS for maximum endurance, precisely where the variometer shows the minimum descent speed. Then multiply that IAS by 1.32 That's the best glide speed. The derivation of 1.32 follows. It was written by DeltaLima for an answer elsewhere on aviation.stackexchange.com.

The ratio is the same for all aeroplanes if you accept a number of
  assumptions:

The propulsion efficiency is constant, regardless of speed or power setting
Aerodynamic drag is the sum of parasite drag and induced drag
Parasite drag is proportional to the square of airspeed:$ D_p = k_p \cdot V^2$
Induced drag is inversely proportional to the square of airspeed: $D_i = \frac{k_i }{V^2}$
There is no wind

Since we assume efficiency is constant, the fuel consumption rate is directly proportional to the power. Power required
  is drag times airspeed: $P = D\cdot V = D_p\cdot V + D_i\cdot V =
> k_p\cdot V^3 + \frac{k_i}{V}$
For the maximum endurance we need to
  minimise the fuel consumption and thus we need to find the speed that
  minimises the power.
$\frac{dP}{dV} = \frac{1}{3} k_p V^2 -
> \frac{k_i}{V^2} = 0$ Solving for $V$ results in $V_{endurance} =
> \sqrt[\uproot{1}4]{   3\frac{k_i}{k_p}} $
For the maximum range we
  need to find the speed that minimises the fuel consumption per
  distance travelled, which is found when the ratio of power to  speed
  over ground is minimal. As we assume there is no wind, the ground
  speed and the airspeed are equal.  Since the ratio of power to
  airspeed is drag, we have to find the speed for minimum drag:
$\frac{dD}{dV} = 2 k_p V - 2\frac{k_i}{V^3} = 0$
Solving for $V$
  results in $V_{range} =   \sqrt[\uproot{1}4]{   \frac{k_i}{k_p}} $
We can now show that the ratio of maximum endurance speed to maximum
  range speed is:  $\frac{V_{endurance}}{V_{range}} = \left.
> \sqrt[\uproot{1}4]{3   \frac{k_i}{k_p}} \middle/  \sqrt[\uproot{1}4]{
> \frac{k_i}{k_p}} \right. =  \sqrt[\uproot{1}4]{ 3} = 1.316... $

Do you have some kind of authoritative source where that 1.32 number came from?

@TannerSwett: Look here for the answer.

Is it possible to calculate it from a speed given by the POH?

In actual practice in some lower-performance sailplanes like Schweizer 2-33 it seems the difference between min sink speed and still-air best glide speed is lower than this calculation would indicate.  Not sure exactly why, or how this answer could best be improved to account for such variations.


text:
What are the differences between air brakes, spoilers and lift dumpers?
So many people use these words interchangeably, but after a little searching on Google, I found they are not quite the same thing. All the links I read explain things in a misleading way which pushed my confusion even further.
I'm looking for a simple explanation on the effect of these parts on The Four Forces of Flight

A short answer, from what I understand, is that air brakes are designed to create drag while spoilers are designed to destroy lift.  In a lot of ways though, the end result (for the pilot flying) is the same in that it helps to slow down or increase the rate of descent.  The only lift dump that I am familiar with is only used on the ground and as the name implies, it kills lift, but is much more effective than a spoiler.

Air brakes increase drag with little change in lift. The following image shows the air brake being used in BAe 146

"Eurowings bae146-300 d-aewb arp" by Adrian Pingstone - Own work. Licensed under Public Domain via Commons.
Spoilers both increase drag and reduce lift- basically, they change the lift-to-drag ratio. Sometimes, they are also used as control surfaces in lieu of ailerons. This image shows a Boeing 777 spoilers being used in flight.

Screengrab from youtube.com
The lift dumpers are basically ground spoilers that are used to 'dump' lift, especially immediately after landing, where you want rapid reduction in lift. The following image shows lift dumpers being used in BAe 146.

Don't the manufacturers themselves even use these terms interchangeably?

In my mind these terms all refer to the same thing with 'Lift Dumper' being a more British/European term. But to help illustrate the source of the confusion a quote from the A300 AMM

The lift dumping function is achieved by control surfaces on the upper
surface of each wing.
In flight, the speedbrake function is achieved on each wing by :

Two inner speedbrakes
Two outer speedbrakes

On the ground, the lift dumping function is achieved on each wing by :

The two inner speedbrakes
The two outer speedbrakes
Three spoilers.

The spoilers are also used in roll control, when they complement the action
of the ailerons.

The A320 AMM

Five spoilers numbered 1 thru 5 inboard to outboard are provided on the rear
upper surface of each wing.
The spoilers are used for:

Roll function
Speedbrake function
Ground spoilers function

So really, the spoiler is the component, while lift dump and speedbrake refer to function the spoiler is providing.

Speed brakes or air brakes is the same word and has the same meaning, mainly is used only in the air. Once the pilot decide to use them 3 or 4 of the spoilers on the wing will deflect little bit but not fully extend. purpose of using it is to 1. Reduce the IAS ( indicated air speed ) 2. To increase rate of descent beacause it will ad ( drag ) when you use them.
Spoiler of also called ground spoilers is only deployed and fully extended once the airplane weight is on the wheel which basically is on landing or when rejecting takeoff. They work by deflecting all spoilers on the wing in an angle that kills lift thus having weight and it helps the airplane decelerate cause you have the whole airplane weight on the wheels

Do you have any references for these definitions? They seem to contradict a bit with the first answer posted.

I fly airbus and these definitons are straight out of airbus manuals. in the begining you should know that the surface that controls everything is called spoilers. In the air spoiler deflection will be called speed brakes used to slow down the aircraft speed or to incrase rate of descent. On the ground it will be called ground spoilers. It will kill lift. Its basic aerodynamic but diff words

Its the same surface but with two functions dependend on where you will use it it will determine the number of spoilers deflected. For example in airbus you have 5 spoilers. When you use speed brakes it uses 3 spoilers when you land and you get the ground spoilers deployed you get all 5 spoilers deployed. In airbus in a turn the airplane uses ailerons and 4 spoilers for turn

Ok thanks for clarifying, that makes more sense. Might be good to edit your answer to clarify that this applies to Airbus, and that the surfaces are all spoilers but the different names refer to the different uses.

This is similar on all manufacturers, for instance on the MD-80 the inboard spoiler is called a GROUND SPOILER and the two outboard are called FLIGHT SPOILERS. On a 737NG, the most outboard and most inboard spoilers are GROUND SPOILERS and the remaining spoilers are FLIGHT SPOILERS.

I don't believe there is any industry standard term for these parts; they're often used interchangeably.  However in general:
Airbrakes and speedbrakes are interchangeable terms; just large reinforced panels which can be extended out from the aerodynamic envelope, creating a lot of parasite drag.
Spoilers and Lift Dumpers are, again, interchangeable terms.  These are just speed brakes mounted on the upper wing surfaces, in general, about 1/2 to 2/3 the length of the chord line.  In addition to creating parasite drag, they induce flow separation over the top of the wing and, as mentioned above, greatly increasing induced drag in the process as the lift is 'dumped' off during deployment.  Large transports will use spoilers to increase braking effectiveness and decrease the length of the landing roll.
Another function of spoilers is roll authority, particularly in fly by wire aircraft, but sometimes seen in other types of mechanically controlled aircraft e.g MU-2, TBM, etc.  They will be used in conjunction with other control surfaces such as ailerons, all-moving tailplanes, etc.


text:
Is it possible to prevent plane accidents due to fuel explosion?
We know that most of the plane accidents and deaths result from the explosion of fuel tanks in the planes, like when two planes collide, or when the plane falls on the ground.
With the advent of technology, i wonder whether it is possible to prevent the fuel explosion.
Can this be done by evaporating the fuel in the planes just before the collision?
Or use a solid architecture encapsulating the fuel tanks in the planes, such that the fuel is not set to fire?
What research has been done in this field and if so what were the findings?

IMHO, the assumption is wrong, we can't say that most deaths are caused by explosion. Hitting the ground is enough reason to die (with or without explosion). It is  the sudden stop on impact, where the plane stops and the person doesn't, at least not right away, that causes severe trauma.

@QuoraFeans technically it's the organs not stopping when the body is stopped that causes the deaths but that's splitting hairs

Yes, there was some technology like this in use, using the fuel evaporation thing before collisions, I think during WW II, worked pretty well... Oh, wait, it wasn't for saving lives...

"We know that most of the plane accidents and deaths result from the explosion of fuel tanks in the planes"
This is a fallacy. Most deaths don't happen because of explosions, but because of impact damage, fire, and inhalation of toxic fumes (in no particular order).
Next to that, many people tend to get killed because they are crushed by fellow passengers during evacuations. And some die on the ground being run over by emergency vehicles or from environmental conditions (heat, cold, wildlife).
Ratchet Freak correctly points out why your "solutions" won't work (in fact one of them will make the problems worse). And yes, research is being done and has been done. Modern jet fuel has a much higher flame point and vapour pressure than does regular gasoline as a result. Fuel tanks are fitted with baffles and sometimes coolers to keep the fuel liquid so it doesn't explode easily. This is in part a result of what happened to TWA800, one of the very few accidents to be caused by a fuel tank explosion (and there the explosion was itself only possible due to a combination of rare circumstances and chance events, made worse by maintenance problems with the aircraft).
The image of aircraft (and cars, trains, ships, buildings, anything really) blowing up in a wall of flame at the least trouble is one created and perpetuated by Hollywood and has no reflection in reality whatsoever.

I think that his question was poorly worded but was asking more about preventing the fuel from catching fire during an accident than an actual literal explosion...

@Lnafziger Yes, you are correct.

@jwenting +1 for mentioning Hollywood. Remembers me of the climax scene in Final Destination 5.

Also, some aircraft can jettison fuel.

@ptgflyer true, but that's not very useful once you're on the ground after a crash. Nor is it useful during a rapidly developing situation like TWA800. The time needed to dump the fuel would in most situations be too long to prevent disaster. It's very helpful though when needing to make an emergency landing while overweight for the runway, which is what those systems were designed for.

And less fire in an off-field emergency landing

@ptgflyer a fuel tank with a lot of fuel vapour in it is more dangerous than one that has mostly liquid fuel in it. Jet-A doesn't burn that easily, but get the right vapour pressure and the mixture becomes rather explosive.

Does dumping make vapor?

@ptgflyer if you dump fuel, you get a fine fuel mist trailing you. But that's not what I meant. If you dump fuel, you create a larger volume of air in the fuel tank, that air can then become saturated with fuel vapour as fuel evaporates inside the tank.

Evaporating/misting is the primary cause of the fireball and adding to that just makes the fireball larger and takes a lot of energy that just isn't available.
NASA tried to study anti misting measures with their controlled impact demonstration once using an additive that made the fuel more syrup-like so it wouldn't spray as much; they essentially made napalm.
A solid structure to contain fuel may sound promising but that would add a lot of weight and is not bulletproof (designing a oblong shape that will survive an 800 km/h impact into concrete is far from trivial).
One option would be to cut the engine+all power and force cool it down using the extinguishing system so there isn't an ignition source to catch the fuel on fire but again isn't fool proof and if there is no collision the plane will be dead in the air.

Indeed, deliberately evaporating a fuel before something ignites it is exacly how a fuel-air bomb works.


text:
Will FIS-B replace onboard Doppler radars?
Will FIS-B technology replace onboard Doppler radars?
These radars can cost upwards from $20,000. Would FIS-B update quick enough to make this a efficient swap?

Not sure where you're going with this one, as ADS-B doesn't have anything to do with weather.

@GregHewgill can't ADS-B receive weather information? Firefight sentry for example.

@Boeing787 ADS-B is a surveillance technology to track aircraft. The onboard Doppler radar provides weather data. FIS-B (Flight Information Service - Broadcast) is a technology that provides weather data. FIS-B is related to ADS-B 978UAT on the technical layer; both use the same transmission frequency and modulation.

Definitely not. ADS-B radar (FIS-B) feeds are from the view of the ground station and have a delay of up to 15 minutes from scan to screen. Onboard radar is obviously instantaneous and even controllable by the pilot for the angle it scans at.
For planes that have onboard radar, FIS-B radar feed doesn't replace what they have. It's a great broader / longer distance situational awareness, but might only change the population of planes that have a radar installed on them at the very lowest end of general aviation.

No. Only onboard radar can give you information about what the weather is like now and near you, as opposed to minutes ago and as seen from the ground. NEXRAD is fine if you are using it for situational awareness and staying well away from bad weather, but for airplanes that are flying close to or directly through bad weather the time delay can mean the difference between life and death.
The NTSB has put out a safety alert on exactly this topic. Sadly, the alert mentions two fatal accidents that were most likely caused by trusting NEXRAD data too much and straying too close to a storm.
Onboard radar has its own limitations, but what data it can provide is nearly instantaneous. Weather data provided through ADS-B and other sources is best used to supplement onboard radar, not replace it.


text:
What is the format specification for Avidyne R9 flight data?
The Avidyne R9 records flight data five times per second and can export it for later analysis.
I'm interested in parsing this data and exporting it to Google Earth / X-Plane for visualization.  Cirrusreports already does this, so I know it's possible.  It would be easier, though, if there was a spec already out there.
Is there a specification for the Avidyne R9 flight data file format available, preferably online?

Can you post a sample file? I can take a peek and see if it's in any of the industry-standard formats I've encountered. Nothing jumped out at me from a quick Google search. (I analyze / reverse engineer data like this all the time.)

According to section 12-4 of the Release 9 Pilot's Guide, data logs are saved as a simple .csv file that can imported into any spreadsheet software (like Microsoft Excel) for analysis.
There are many free tools that can covert Excel files to KML for Google Earth, ie: http://www.earthpoint.us/exceltokml.aspx


text:
What is "Carson cruise" and can I determine it myself?
Mike Y. just posted an answer containing the following:

With today’s fuel costs, the Carson cruise angle of attack, which represents the most efficient way to fly fast with the least increase in fuel consumption, is of significant interest.

Being an outstanding human, Mike also added a reference to the document that it came from:

"Low-Cost Accurate Angle-of-Attack System", Borja Martos and David F. Rogers, Journal of Aircraft 2018 55:2, 660-665

But for those of us who don't have access to that journal or can't be bothered to go find it, what is Carson cruise?  What exactly are the benefits and how is it determined?  Finally, can I determine it myself for my airplane without an AOA indicator?

From this highly reliable web page http://www.eaa1000.av.org

source
The maximum range airspeed may be the most economical way to fly, but it's quite slow... Hence, we may consider the speed for which the ratio fuel flow/knot of airspeed reaches a minimum. That speed, marked by the point where a line from the origin is tangent to the thrust required curve, is called 'Carson's speed'.

source
In s/l flight, drag = thrust. Hence, the lower graph shows the 'thrust required' curve. There is a fixed ratio (1,316) between the maximum endurance speed, maximum range speed and Carson's speed, so that:
Maximum range speed = 1,316 x Maximum endurance speed
Carson's speed = 1,316 x Maximum range speed
That's a simple way to find Carson's speed for your plane. You can get the value of the Max. range speed from the POH

Can you summarize the page in your answer? Images aren't searchable and it's too small to read easily on mobile.

@Pondlife  Done...

Isn’t TAS/FF the specific range? So wouldn’t Carson cruise simply maximise specific range? In a jet, that’d be max range speed, right? What am I missing?

@CptReynolds I believe you are correct, but I have seen the use of Carson's speed most commonly in general aviation where pilots want to fly faster than max range speed for a propeller airplane, for which this 'Carson's speed' is the best compromise.

Isn't this just a heuristic? If fuel was everything you would fly at the maximum range/minimum fuel per km speed. But crew wages, plane depreciation, and passenger patience are other consumables which vary from flight to flight. Factor these in and there no simple formula for "best" cruise speed.


text:
How to safely wash a Nomex flight suit?
Flight suits made of Nomex are flame retardant, which is important for flying some kinds of aircraft. However, they're not oil, dirt, or sweat retardant, so sooner or later they need a wash - a wash strong enough to remove all that muck.
Is there anything special you need to do to get them clean without losing their flame retardant quality?

If only there was a AutoRacing.SE, I'm certain you would get an answer there...

The Nomex cleaning products from Molecule Labs seem to be recommended.

Firstly to clarify:

What are the differences between “inherent” and “treated” flame resistant fabrics?
Inherently flame resistant fibers are materials that have flame resistance built into their chemical structures. Aramid fibers from companies like DuPont create inherently flame resistant fibers, with brands such as DuPont™ Kevlar® and DuPont™ Nomex®.....

With that information:

Laundering’s impact on flame resistance
An essential thing to keep in mind when assessing flame-resistant technologies is that inherent flame resistant properties cannot be washed out or damaged through exposure to chemicals in the workplace or laundering practices, whether at-home or commercial.
FR-treated garments, however, may be damaged by chlorine bleach, the combination of hydrogen peroxide (“oxygen bleach”) with hard water, or exposure to oxidizing chemicals in the workplace. Source

An extensive DuPoint Nomex Laundering Guide
For the 'treated' fabrics, reading this DuPont document might be interesting.

So essentially, cleaning a Nomex flight suit will make it more fire resistant, since you're washing out flammable dirt...

It would help if you added a quick summary of the DuPont (not DuPoint) Laundering guide, like saying that "DuPont recommends washing Nomex separately using heavy duty detergent in soft water to ensure that the washed suit is completely free of any flammable lint, dirt, or residue.  If necessary Nomex can be ironed at a medium setting"


text:
What are Mode C requirements with ADS-B 978?
ADS-B out is available on the 1090 (Mode S) band, and can even be implemented as a firmware update to some Mode S transponders.
In the case of 978 ADS-B Out, I believe that it is still necessary to keep operating a Mode C transponder for operations where Mode C is currently required, as noted in This video at 16:25, in order to work with TCAS systems.  Continuing on to 41:24, there is a recommendation stating that if a Mode C transponder fails, it should be replaced with an ADS-B "out" transponder.  Here are my questions:

Is there such thing as a 978 transponder which will meet ads-b out requirements and eliminate the need for Mode C, or does this recommendation essentially mean that you move to a 1090ES transponder?
Perhaps this recommendation is assuming that there was no ads-b out at the time.  Given an aircraft with a failed mode-c transponder, what equipment options which meet the mandates are worth considering for GA aircraft?  Apparently John Zimmerman does not recommend Mode C + 978 out.  It appears he recommends 1090ES.  Either one of these would seem to benefit from having 978 in.  I can see no advantage to 1090ES in, as it appears to offer only a subset of the 978 services, unless it was desired to be able to receive 1090ES air-to-air or our operate outside the US.

Even with ADS-B, the rules for entering class B and C airspace don't change, you still need a working Mode-C transmitter.

For Class A,B and C, you'll need a Mode C transponder as per the present regulations (unless cleared by ATC). From 14 CFR §91.215 ATC transponder and altitude reporting equipment and use:

§91.215   ATC transponder and altitude reporting equipment and use.
(b) All airspace. Unless otherwise authorized or directed by ATC, no person may operate an aircraft in the airspace described in paragraphs (b)(1) through (b)(5) of this section, unless that aircraft is equipped with an operable coded radar beacon transponder ... and that aircraft is equipped with automatic pressure altitude reporting equipment having a Mode C capability that automatically replies to Mode C interrogations by transmitting pressure altitude information in 100-foot increments. This requirement applies—
(1) All aircraft. In Class A, Class B, and Class C airspace areas;

FAA also addresses this in its FAQs regarding ADS-B:

Will Mode C transponders be required indefinitely?
The FAA is considering additional changes in the national airspace system, such as for TCAS. These changes, may at some future date, reduce or eliminate the need for transponder equipage.

So, at present, you'll need Mode C anyway.
Also, you'll need 1090ES in Class A. From FAA FAQs:

Aircraft that fly internationally and aircraft that require TCAS (Traffic Collision Avoidance System) are already operating with Mode S transponders and many have older versions of ADS-B Out on 1090ES MHz. Aircraft flying in Class A airspace must operate on the 1090ES MHz frequency. General aviation users who choose the UAT (978 MHz) link may take advantage of the ADS-B traffic, weather, and aeronautical services that are transmitted on the UAT frequency at no charge but must retain their current ATCRBS transponder. Mode S, 1090-ES users can operate in all airspace, but cannot receive FIS-B services.

I agree with you that there are a number of services (through FIS-B) for 978 UAT. In that case, (if you are not going above 18,000 ft) I suggest you check the availability of UAT receivers before taking a decision.

1.) Not at this time, but someday maybe. It is reasonable that transponders will someday become obsolete with the advent of NextGEN infrastructure (including ADS-B) being implemented worldwide.
The transponder lends itself to a radar based ATC system for determining 'who' and 'where' aircraft are located. Although the radar can determine a 'where', the transponder fills in the 'who'.
NextGEN is planning to do away with radars (for the most part). ADB-S (a major component of NextGen) can provide ATC both the 'who' and the 'where' (and then some) on its own (no radar needed).
It could be years or decades before transponder requirements are relaxed however.
2.) '978 in' or 'ADS-B in' is a compelling reason for installing a UAT. I'd wager that even aircraft meeting the 2020 mandate with a 1090ES mode S transponder will eventually install UAT receivers. One advantage, however, to 1090ES is it is the only recognized ADB-S equipage anywhere outside of the USA at this time.


text:
Is it necessary to have my full name (including middle name) on my student pilot license?
So my student pilot license is missing a middle name, but my drivers license, written exam, and passport all have it printed.
I've spoken with the DPE, and he says it may be an issue, or it may not, and that he is going to look into it.
I've called FSDO, airmen certification branch, and other DMVesque organizations, and they all seem to say the same thing:

Oh, you need to talk to (insert amorphous bureaucracy here)

Does anyone know who I actually need to talk to, or if this is even an issue? Do I just need to call FSDO and insist that they fix the issue, or what?
The DPE said that with the written exam being OK it may be a non-issue. Would love to hear from a DPE on here if possible.

The FAA people must not be paying much attention lately. This is the 3rd licence discrepancy question on SE in two months

Contact the person who issued it to you Ie. The AME/DPE/CFI etc.

@RyanMortensen the AME no longer practices...

@OneChillDude I had the same issue. My passport doesn't have a middle name field, and includes my middle-name as part of my first-name. My DPE noticed this and hence left the middle-name field blank on IACRA, and followed what my passport had. So, I believe the Passport/Birth Certificate holds more value when identifying yourself.

@OneChillDude ouch.  I suppose the real question is whether anything official says that your SPL/medical must have a middle name on it.  Maybe contact the Civil Aerospace Medical Institute in Oklahoma City.  They are the authority on medicals.

I have a meeting with FSDO on Wednesday. I'll report back soon

Alright so here is the outcome:
I called FSDO, spoke with a real nice guy who told me to give him a call if I couldn't handle this online.
Called a zillion bureaucracies, all telling me to call someone else.
Called the nice guy at FSDO, set up an appointment for a name correction not a name change. Went to the appointment and was in and out in under an hour.
If you're having the same issue, I would recommend bringing every official identification you've got: passport, birth certificate, drivers license, etc.
Tl;DR call FSDO and tell them you need a name correction. NOT a name change. Can't stress that enough.

Thank you for sharing the outcome!  Did you happen to learn whether or not your AME could have helped you if he/she were still practicing?

@RyanMortensen I did not find that out, however I am 99% sure that the AME can reissue a medical, however that will cost money, FSDO is free

Just ran into this issue myself (less than a week before PPL checkride of course). In the time since this question there has been a clarification to this exact scenario (middle initial on the student license). https://dperesources.com/clarification-on-name-mismatches-from-afs720-middle-name-middle-initial-discrepancy/
If I'm reading it correctly this indicates that you don't need to get a new license, but you still need to make sure that your middle name on the checkride application is your full middle name and not your middle initial (hopefully I can get the FSDO to go and change it tomorrow when they open).


text:
Is it allowed to log actual instrument under IFR in VMC when VFR would be prohibited?
Flying with an IFR clearance near IMC but with visual references where VFR would be prohibited I have the following questions:

Are you technically even in VMC if you are that close to a cloud? Technically flight is not strictly by reference to instruments, but VFR would be illegal.
Can an instructor log instrument time? (a PPL would not be allowed to fly through tiny holes in a cloud)
Does an instrument student need foggles or can it be logged as "actual"?
Are you still required to see and avoid (obviously still a good idea)

Example: you are 100ft above a cloud and 1000ft horizontal in class E airspace under IFR. You can technically see but need the IFR clearance to be legal.

Outside of Class B and C, ATC is responsible for keeping IFR traffic separated from other IFR traffic. If you are in VMC conditions they are not responsible for separation from VFR traffic—that’s where 'see and avoid' comes in.

The main source of confusion here is the difference between VFR/IFR and VMC/IMC.  Logging actual instrument time requires you to be in IMC, regardless of the rules under which you operate (even if it would be illegal to do so when VFR).

VFR and VMC are entirely separate. Clear day at FL 250, you can't fly VFR but you're in VMC. Moonless night over water with a high cloud deck, you might be legal to fly there VFR but without a horizon you're effectively in IMC. Don't log the former as instrument time; your discretion how you want to log the latter.

@RalphJ This is kind of a pub quiz thing but you can fly VFR at FL250 in the US. You just have to be in Hawaii, there's no class A airspace there :-)

@Pondlife Actually, 91.135 offers deviation authority to fly VFR in Class A airspace with permission from ATC, so it doesn't even have to be in Hawaii.  ;-)

Have flown glider in Class A airspace, with an ATC clearance. The glider lacked the instrumentation necessary for an IFR clearance, so a VFR clearance was obtained.

14 CFR 61.51(g) says (my emphasis):

(g) Logging instrument time. (1) A person may log instrument time
  only for that flight time when the person operates the aircraft
  solely by reference to instruments under actual or simulated
  instrument flight conditions.
(2) An authorized instructor may log instrument time when conducting
  instrument flight instruction in actual instrument flight
  conditions.

Based on that, the answers to your questions are:

Are you technically even in VMC if you are that close to a cloud? Technically flight is not strictly by reference to instruments, but VFR would be illegal. You're in IMC (i.e. you're operating below the visual minimums in 91.155) but as you said, that doesn't mean you're flying "solely by reference to instruments"
Can an instructor log instrument time? (a PPL would not be allowed to fly through tiny holes in a cloud) If the conditions are IMC, yes (61.51 doesn't require an instructor to operate the aircraft using only instruments)
Does an instrument student need foggles or can it be logged as "actual"? Whether you're wearing foggles or not, you can log instrument time only when you're operating solely on instruments. Practically speaking, using foggles guarantees that you're doing that and can therefore log the time. Without them, some or all of the time might be loggable, i.e. only the part that you really spend flying only on instruments.
Are you still required to see and avoid (obviously still a good idea) Yes, absolutely. According to 91.113(b):

When weather conditions permit, regardless of whether an operation is
  conducted under instrument flight rules or visual flight rules,
  vigilance shall be maintained by each person operating an aircraft so
  as to see and avoid other aircraft.

It's very common to fly cross country under IFR without logging any instrument time, or logging very little, simply because you're never solely on instruments.

So really there are three distinctions: flight rules, meteorological conditions, and then the phrase solely by reference to instruments, and a pilot could be in many valid combinations, i.e. VMC under VFR, VMC under IFR, IMC under IFR, and flight could be "solely by reference to instruments" in any of the three.

@OneChillDude I can see most other combinations, but one: can you give some example of how can one be "solely by reference to instruments" in "VMC under VFR"?

@Suma See point 3 in my answer. If you're wearing a 'hood' or other device that means you can't see outside the aircraft, then you're solely on instruments. That's how most instrument training works. There are other possibilities too: a very dark night with no ground lights or horizon reference; a night takeoff over the sea etc. In all those cases you can be VFR in VMC but you still need to rely completely on instruments to fly the aircraft.


text:
What are the wake turbulence characteristics at ground level near landing aircraft?
I'm a structural engineer and I need to understand the additional forces that wake turbulence may cause on a building near a runway.
I can determine the natural wind speed and associated forces on the building, but I need to calculate the additional wind forces/ turbulence (or airflow velocities) caused by aircraft operating on a nearby runway.
The runway is 3,400m (11000 ft) long, and the position of the building is 800 meters along the extended runway centreline and 300 meters to the side of it. These measurements are from the very end of the paved surface (which I will have to assume to be the first/ last possible contact point from the aircraft for the design of the building). The runway is approached by (and departed from) that end.
How do I calculate the airflow velocities at ground level below an aircraft (assume an Airbus A380) at the building's location? The surrounding ground is flat and level.

This would certainly help. I guess I also need to work out the rate of rise of the aircraft, so that I can calculate the height of the aircraft 800m from lift off (or touch down!)

I'm not capable of answering your question, but some clarification could be helpful to others who are capable. "Wind speed" refers to natural wind (weather).  Are you asking about how to calculate the airflow velocities created by a departing aircraft's wake turbulence (wing tip vorticies and downwash)?

Oddly specific, but still too vague:  Which end of the runway?  How long is the runway?  Aircraft weight and runway length will affect altitude as it flies over.  How accurate do you want the answer?   And what are you actually trying to do, learn the process of calculating it, or looking for an estimate of something you experienced the effect of?  It might be easier to just measure it…

Hello Andy Leigh, welcome to aviation.stackexchange. I took some of the information you provided in the now deleted answer and merged it into the question. I also did a general restructuring of the question, to the best of my understanding of its intention. Can you check whether I have captured the essentials and adjust if needed?

Departing aircraft do not lift-off at the last possible moment, and they climb steeper that landing aircraft descent. Therefore they are higher up when they pass the building. I think the critical case will be the landing.

Can you say which airport / runway it is? Some runways have a displaced threshold, meaning the aircraft will be higher when they pass the building. Also approach path is not equally steep at all runways (I would assume 3 degrees here)

My educated guess would be that, (depending on your area) local building codes for natural wind will be sufficient.  At the distances you specify I cannot imagine aircraft wake exceeding the gust from a strong storm.  But to my earlier comment, it might be easier to measure, or even estimate just by the personal observation of standing there when a plane flys by.  If the blast doesn’t knock you over it probably won’t harm your building either…

@MichaelHall I agree that the standard storm wind load should be sufficient. For the case that an aircraft lands in a storm I would not be worried about the combined effects, as wake turbulence vortices dissipate quickly if the air conditions are turbulent by itself due to the storm.

Unfortunately I can't say which airport/ runway, due to client confidentiality rules. Thank you for adjusting the question. I will be reviewing the documents that you have suggested. We have built close to the current site, and there determined that the natural storm forces were an over-riding criteria, although we applied them from all directions, rather than just from the prevailing wind direction, and we have used this approach for the scheme design. There is a building on the site currently, and as soon as I am able to travel, will review the structure there and inspect for distress.

I can't really answer the  question with accurate numbers, but have a look at these studies:

Reducing turbulence near airports - DLR tests new
procedures to mitigate wake vortices- DLR 2013
Mitigating Wake Turbulence Risk
During Final Approach via Plate Lines - DLR 2020

They may be a starting point for finding an accurate answer.
At Frankfurt international airport, they have constructed a new runway about 10 years ago. There are buildings at the same relative position as yours, (800 meter longitudinal, 300 meter lateral) that were there before the runway was constructed. As far as I know, that does not pose any problems with wake turbulence. However, this runway is not used by the A380 and B747 (but it is used by B777 aircraft)


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Where can I get an list of ATC phraseology terms in Spanish?
I'm considering flying in a Spanish-speaking country and would like to have a passing familiarity with how to communicate with ATC in Spanish. Is there an ATC English-to-Spanish dictionary?

This doesn't really answer your question, but are you sure you want to talk to ATC in Spanish? If you aren't a fluent Spanish speaker, you might be better off just speaking English to them (which they're generally required to be proficient in.) IIRC, speaking Spanish to them gives them implicit permission to send further transmissions to you in Spanish, rather than English. Of course, understanding Spanish phraseology could still be helpful for understanding what the locals are saying to ATC for situational awareness purposes.

@reirab is completely right, if you can't speak fluent spanish, and in the local dialect and accent you are probably better off using English. I've not flown into Spain, but I have to France and I couldn't understand a word, even though I'd studied the phrases.

ICAO has defined in their Annexes that English is only one of the available langauges to ICAO and that ATS should either use the language specified in the country's AIP or use English.

1.2.9    Language proficiency
1.2.9.1    Aeroplane, airship, helicopter and powered-lift pilots, air traffic controllers and aeronautical station operators
  shall demonstrate the ability to speak and understand the language used for radiotelephony communications to the level
  specified in the language proficiency requirements in Appendix 1.
Note 2.— The provisions of 1.2.9 refer to Annex 10, Volume II, Chapter 5, whereby the language used for radiotelephony
  communications may be the language normally used by the station on the ground or English. In practice, therefore, there will
  be situations whereby flight crew members will only need to speak the language normally used by the station on the ground.

Personally, additional to my proficiency level in English, I also hold a Level 6 language proficiency German in my pilot's license.
Below is a screenshot of the Aerodrome Description for LEBL/Barcelona, taken from Spain's AIP. You see that ES/EN is listed as available languages.

If you are looking for Spanish Phraseology, you can find it with Argentina's Administracion Nacional de Aviacion Civil.


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Do military tactical transport flights provide seatbelts for all occupants?
In several movies I have watched recently, the temporary bench style seats of tactical transport aircraft such as the C-130 do not show seat-belts. I would not be surprised that some instances of seat belts would be ignored during times of war but let's limit this question to standard operations.
So, is it standard practice for transport aircraft such as the C-130 or Chinook helicopter during large personnel missions to provide all occupants with seat belts?

Every one I've been on has had them going back to the 70's. Here's a picture that clearly shows them:

Source: Wikimedia